The present invention concerns a method of improving the linearity of a photo detector of the kind in which electrical signals are used to define the position of a point illuminated on a detector surface included in the detector, said detector comprising a semiconductor plate, two resistive layers each one covering one side of the semiconductor plate, a p-n junction separating the two resistive layers and two pairs of electrodes arranged one at each resistive layer and which electrodes are of such a shape and are so positioned that each one of the two pairs defines two opposite sides of a right-angled square, said square defining two portions of the resistive layers which portions are positioned in opposite and parallel relationship.
Lateral photo detectors of the kind outlined above suffer from the disadvantage that the connection between the output signals and the coordinates relating to the position is non-linear, particularly close to the margin of the detector surface defined by the electrodes. For this reason it has unfortunately hitherto been necessary, in cases where the demand on linearity is strict, to limit oneself to the use of a minor portion of the detector surface positioned symmetrically relative to the centre thereof where the edge effects are not very noticeable. For instance, by giving to the electrodes a particular geometrical configuration, more precisely by allowing their ends to extend across and beyond the electrodes positioned at the opposite side it becomes possible to affect the edge effects to some extent by so to speak displace the margin, but in order to achieve real efficiency in this way the electrodes must be made of such a length that the detector becomes uncomfortably large in relation to the useful detector surface. In addition, the area of the p-n junction and the circumference of the semiconductor plate increases, which gives rise to a non-desirable increase of the leakage current through the p-n junction and over the margin thereof, with subsequent noise increase and of course a necessity for larger amounts of materials.